1. Field of the Invention
The present invention relates to a system and method for substrate detection and, more particularly, to substrate eccentricity detection.
2. Brief Description of Related Developments
Typical manufacturing processes for semiconductor integrated circuits may utilize robotic manipulators to cycle substrates, for example, circular silicon wafers, through pre-determined sequences of operations in fully automated processing equipment. Substrates may be delivered to the substrate processing equipment, also referred to as a tool, in standard transportation cassettes which house a batch of substrates stored in horizontal slots. Individual substrates may then be transferred from the cassettes by a specialized pick-place robot which may be integrated into the tool. Typically, the robot holds a substrate by means of frictional force between the backside of the substrate and an end-effector. In some applications, the force may be supplemented by a controlled suction-cup gripper.
As a result of limited, but not negligible, motion of the substrates in the cassettes during transportation, the robot may pick the substrate with undesirable eccentricity or misalignment. The difference between the actual location of the center of the substrate and the specified position on the robot end-effector needs to be corrected before the substrate can be processed in the tool. Existing methods and devices for determination and correction of eccentricity or misalignment of circular substrates may include stationary aligners, aligners built into the robot end effector, and external sensors.
When utilizing a stationary aligner, a robot places the substrate on a chuck of a stationary rotating device which rotates the substrate while scanning its edge for a fiducial location and substrate eccentricity. The aligner then moves the substrate to a centered position or transmits the resulting eccentricity vector to the robot which utilizes this information to pick the substrate in a centered manner. This approach introduces undesirable delays associated with the additional pick-place operations and with the edge-scanning process, all of which are executed sequentially rather than in an on-the-fly manner. It also undesirably increases the overall complexity and cost of the system.
An aligner may be integrated into the robot end-effector that mechanically centers the substrate and then scans its edge for fiducial location. The aligning process may take place on the fly during a regular substrate transfer operation, which can improve throughput performance. However, the mass and complexity of the moving components of the robot arm increases undesirably, which results in limited speed, compromised reliability and a higher cost.
Determination of substrate eccentricity using external sensors generally includes moving the substrate through a set of sensors which detect the leading and trailing edges of the substrate. The resulting information is processed to determine the actual location of the center of the substrate. The alignment process takes place on the fly during regular substrate transfer operations without increasing the mass or complexity of the robot arm. One example of the use of sensors for determining substrate eccentricity is disclosed by U.S. Pat. No. 5,706,201, issued on Jan. 6, 1998 to J. Randolph Andrews, entitled Software to Determine the Position of the Center of a Wafer. However, one disadvantage of this method is that it requires an array of multiple sensors.
It would be advantageous to provide a system for determining eccentricity or misalignment that includes a limited number of sensors to reduce cost and also overcomes the above mentioned disadvantages and other shortcomings of presently available systems.